docs(github): add pull request template

- add PULL_REQUEST_TEMPLATE.md with what/details/checklist sections
- checklist adapted to this repo's build, link-check, and llms.txt conventions

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Yejin Kelly Joo <yejinkellyjoo@gmail.com>

Author: yjkellyjoo

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,17 @@
+# What does this PR resolve? 🚀
+
+<!-- One-line summary. Then bullets: the key changes and, briefly, the why. -->
+
+# Details 📝
+
+<!-- Implementation specifics, trade-offs, anything a reviewer needs. -->
+
+# Checklist ✅
+
+- [ ] Merged latest `master` and resolved conflicts
+- [ ] `npm run build` succeeds
+- [ ] Links checked (`npm run check:links`) where relevant
+- [ ] `static/llms.txt` updated if pages were added / renamed / deleted
+- [ ] Content follows [CODING.md](../CODING.md) conventions (wrap long lines, `Swarm` vs `swarm`)
+- [ ] Self-reviewed the diff
+- [ ] Commits are signed off (`git commit -s`)